This invention relates to a coating fluid for the preparation of an ink absorbing recording medium for use in inkjet printing, to a method for the preparation of the coating fluid, to a an ink absorbing recording medium for use in ink jet printing and to a process for the preparation of the ink absorbing recording medium.
In recent years reflecting the use of electronic still cameras and computers, the hard copy technology to record images on paper sheets has been rapidly developed. The goal is to bring the colour reproducibility, image density, gloss and weather resistance of the hard copy to the same level as silver halide photography. Ink jet systems have been used for hard copy recording since full colouring is easy and the printing noise is low. In an ink jet system ink droplets are ejected at high speed from nozzles to a recording material and the ink contains a large amount of solvent which is usually water.
The recording sheet for an inkjet printer is required to swiftly absorb the ink and have good colour forming properties. To reduce the dry time of the ink it has been previously proposed to provide a recording sheet having a porous layer formed on a substrate, the porous layer being formed of an inorganic oxides such as aluminium oxides or silica.
European Patent Application No 586 846 discloses an ink jet recording sheet having an ink receiving layer containing a cation modified non spherical colloidal silica. By non spherical is meant substantially non spherical and includes acicular and fibrous in shape. The cation modified silica may be prepared by coating the colloidal silica with a hydrous metal oxide such as aluminium oxide.
The acicular and fibrous cation modified silica described in the above mentioned European Patent Application No 586846 are relatively expensive and there is an economic incentive to use a silica which has not been cation modified where possible.
Silica which has not been cation modified usually contains surface hydroxyl groups capable of releasing hydrogen ions and leaving a negative charge on the surface. Such silica is referred to in the present specification as anionic silica. In inkjet printing it is generally preferred to produce prints which are glossy rather than matte in appearance. However it is more difficult to obtain glossy prints from anionic silica than from cationic silica. Glossy products have been previously produced from anionic silica but this has often required the use special coating techniques such as cast coating.
The present invention provides a solution to this problem by providing a novel coating fluid comprising an anionic inorganic oxide selected from silica and alumina and a solution of a composition containing a cationic surfactant and a polymer containing negatively charged groups. Many of these novel coating fluids are able to produce a glossy print without the need of cast coating. Others of the novel coating fluids produce matte coatings which are nevertheless useful in many applications.
According to the present invention there is provided a coating fluid for forming a coating on a support for use in printing said fluid comprising an aqueous liquid having dispersed therein
(a) from 2 to 50% of an anionic inorganic oxide selected from aluminium oxide and silica the % being by weight based on the combined weight of aqueous liquid and inorganic oxide and
(b) from 1 to 50% by weight based on the weight of oxide (a) of a composition containing a polymer containing negatively charged groups and a cationic surfactant.
According to another aspect of the present invention there is provided a process for the preparation of a coated support for use in inkjet printing which process comprises applying to the support a coating fluid as hereinbefore defined and allowing the coating to dry.
According to a further aspect of the present invention there is provided a support having a coating formed from a coating fluid as hereinbefore defined.
The advantage of the present invention is that it provides coatings which can be used in inkjet printing to yield glossy prints without the need of special coating techniques.
Although the coatings of the present invention are particularly suitable for use in ink jet printing, they may also be used in other printing applications e.g. as supercoats on photographic papers, or as part of a colour diffusion transfer receiver sheet.
Furthermore although glossy coatings are preferred, it is intended to include within the scope of the present invention matte coatings which have been prepared from the anionic inorganic oxide and composition containing anionic polymer and cationic surfactant as defined above.
The term cationic surfactant in the present specification is intended to include molecules which contain, in addition to a positively charged atom such as quaternary nitrogen, one or more hydrophobic groups such as butyl, pentyl, hexyl or heptyl.
The term therefore includes compounds such as tetrabutyl ammonium chloride which are not usually classified as surfactants.
The cationic surfactant should be one capable of forming water soluble complexes with maleic acid copolymers and may for example be one containing quaternary nitrogen for example quaternary ammonium or pyridinium for example cetyl trimethyl ammonium chloride, or cetyl pyridinium chloride.
Preferably the hydrophobic portion comprises a hydrocarbon group containing more than 4 carbon atoms, more preferably at least 8 carbon atoms although quaternary compounds bearing high proportions of hydrophobic groups, for example didodecyl dimethyl ammonium bromide may not form water-soluble complexes with maleic acid copolymers and so will not be suitable for use in the present invention. Benzyl tributyl ammonium chloride may also be used but it has been found that the product is not glossy. Coating fluids containing this surfactant are however particularly suitable for use in a lower layer of a recording medium.
Preferably the amount of inorganic oxide is from 3 to 30% by weight based on the combined weight of aqueous liquid and inorganic oxide and preferably the amount of complex is from 2 to 20% based on the weight of the inorganic oxide.
The liquid medium in which the inorganic oxide and polymer complex are dispersed may be water. It is possible to employ other solvents in addition to the water but it is preferred to employ water alone.
A binder polymer may also be included in the coating fluid although the coating fluids of the present invention do not require a binder polymer and it is preferred not to have one present. If one is employed, the binder polymer may be any of those known in the art. Such polymers include polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetate, copolymers of styrene and acrylates, copolymers of styrene and butadiene, polyacrylates and polymethacrylates, copolymers of acrylic and methacrylic acid, polyacylamides and polymethacrylamides.
The inorganic oxide should be dispersible in water and is preferably capable of forming a colloidal dispersion in water.
Preferably the inorganic oxide is silica.
The forms of silica and alumina suitable for use in the present invention are those having surface hydroxyl groups which are capable of releasing a hydrogen ion to leave a negative charge on the surface. These forms of silica and alumina are referred to in the present specification as anionic.
The silica may be one that is known in the art as monodisperse or polydisperse. When the inorganic oxide is aluminium oxide it usually comprises aluminium oxide hydrate particles. These are capable of being dispersed to form a colloidal dispersion in water.
The aluminium oxide is preferably fumed or colloidal since the particles are smaller (less than 100 nm) although other forms such as precipitated or gel may be used.
As the aluminium oxide hydrate one represented by the compositional formula Al2O3.nH2O where n is from 1 to 1.5 may be used. It is suitable as an ink receiving layer since the colourant is well absorbed and fixed to it. Aluminium oxide in the form of Bohemite or pseudo-Bohemite may be used. The latter is described in U.S. Pat. No. 5,104,730.
The polymer containing negatively charged groups hereafter referred to as the anionic polymer is preferably one containing carboxylic acid groups on two adjacent carbon atoms in the polymer chain with a spacer group provided by a comonomer separating pairs of carbon atoms bearing the carboxylic acid groups.
The anionic polymer conveniently has a molecular weight in the range from 5,000 to one million and is preferably a copolymer obtained by copolymerising a maleic acid containing monomer with another ethylenically unsaturated monomer or monomers and where the molar ratio of maleic acid containing monomers to other monomers is from 1:1 to 1:5.
The maleic acid may be provided by its esters or maleic anhydride. Examples of the other monomers are methyl vinyl ether, styrene, isobutylene, and acrylic acid.
The coating fluid may be prepared by mixing together aqueous solutions of polymer containing maleic acid units and cationic surfactant, stirring until any precipitate which may form has dissolved and then mixing the resultant solution with a colloidal silica dispersion.
The combined mixture is then coated on the support.
The relative amount of anionic polymer to cationic surfactant should be sufficient to maintain the composition in solution or where a precipitate is formed to redissolve the precipitate.
The relative quantities can vary with the structures of the cationic surfactant and maleic acid copolymer used, but generally it is found that a molar ratio of cationic surfactant to maleate ion units of between 2:100 to 1:1 is suitable that is the molar ratio of cationic surfactant to carboxylate or carboxylic acid units of between 1:100 to 1:2.
Suitable colloidal silica dispersions bear negative charges on the silica particles, and may be of relatively uniform particle size as in Ludox silicas available from Du Pont, or polydisperse as in Syton silicas from the same supplier. Suitable particle sizes for the colloidal silica range between 10 and 100 nanometers in diameter. In addition particles of diameter up to about 2 nanometers may be used although these may not be colloidal.
The quaternary salt-maleic copolymer complex is preferably present at between 1 and 20% of the silica present, expressed as dry weight.
Other substances may be included in the coating fluid for example other polymers, latexes, inorganic particles including aluminium oxides, clays and minerals and metal ions.
Sufficient microporous silica may be coated on a support to absorb all the ink applied by an ink jet printer. That requires an ink absorption capacity of approximately 10 to 20 ml per square meter that may require a silica coverage of about 50 g/square meter.
Alternatively the silica layer may be applied as a coating above or below another ink absorbent layer, for example water absorbent paper, or a layer of gelatin or other water absorbent polymer in which case lower silica coverages of between 1 and 20 g/square meter may be suitable.
It has been found that some of the coating fluids of the invention gel rapidly. Therefore according to one embodiment the method of coating is carried out by forming two streams, the first comprising the aqueous dispersion of inorganic oxide selected from silica and alumina, the second stream comprising an aqueous dispersion of the composition comprising polymer containing negatively charged groups and cationic surfactant, and mixing the two streams and immediately applying the mixture to the support.
The invention is illustrated by the following Examples.